Why under voltage tripping of motor is incorporated in motor feeder breakers?

Why under voltage tripping of motor is incorporated in motor feeder breakers? 
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Answer :

The under voltage can occur in case of bus fault. If the motors are kept connected they will feed the fault which may cause the damage. Due to the back feeding from the motor the motor will slow down very fast. Hence process system will come to halt very fast. (In case pht motor will not rotate for the designed 3 minute period in case of bus under voltage).  
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